1. Field of the Invention
The present invention relates to sonar, and more particularly, to an apparatus and method for side look sonar.
2. Description of Related Art
In sonar systems, acoustic energy is emitted toward a target field which is comprised of numerous scattering points. Reflected energy or echos from these scattering points is detected by a receive array including of one or more receiver elements. The output signals from the receiver elements are proportional to the reflectivity response of the scatters as the energy propagates across the target field. In side looking sonar systems, energy is emitted in a direction normal to an underwater platform's direction of travel. By emitting pulses at regular time intervals as the underwater platform advances, a two-dimensional reflectivity map of the target field may be collected over time. The resulting two dimensional map may be detected and processed to form an image of the ocean bottom for example.
Generally, the performance of side looking sonar imaging systems is governed by resolution in both the direction of platform movement, herein referred to as along track, and normal to platform movement, herein referred to as cross track. Resolution determines the degree to which signals of interest, such as targets, are separable or in other words detectable from bottom reverberation. Improved resolution minimizes the patch size intercepted in along track and/or cross track dimensions. The smaller the patch size, the better the detection performance because the bottom reverberation with which target returns must compete is reduced. Along track resolution is determined by the horizontal receive array aperture. A fixed horizontal aperture defines a response function that is fixed over an angular extent, that is, beamwidth. Thus, as range increases, the along track direction of the patch from which acoustic energy is received grows in the along track dimension thus reducing resolution.
Cross track resolution is determined largely by the transmit pulse width at long ranges. At ranges closer to the platform, the pulse intercepts the bottom with a higher grazing angle, causing the dimension of the cross track patch width to grow relative to long ranges. A higher grazing angle and larger patch size increases the bottom reverberation that is returned to the receiver. Thus, targets located at close in ranges are therefor more difficult to detect than those at far field ranges since they compete with higher background signals. Because of this limited performance, the region near the platform is referred to as the "gap."
In light of the foregoing, there is a need for a side looking sonar system and method that eliminates the gap without sacrificing the resolution in the along track direction.